According to the increase of technical development and demand on mobile devices, the demand on secondary batteries as an energy source has been rapidly increased. Among the secondary batteries, lithium secondary batteries having high energy density and voltage are commercially available and widely used.
As a positive electrode active material of a lithium secondary battery, a lithium metal oxide is used, and as a negative electrode active material, a lithium metal, a lithium alloy, crystalline or amorphous carbon or a carbon composite is used. The active material is coated on a current collector to an appropriate thickness and length, or the active material itself is coated as a film shape and then is wrapped or stacked with a separator that is an insulating material, to form an electrode group. After that, the electrode group is inserted in a can or a vessel similar thereto, and an electrolyte is injected therein to manufacture a secondary battery.
In the lithium secondary battery, lithium ions repeat intercalation and deintercalation from a lithium metal oxide of a positive electrode to a carbon electrode to conduct charging and discharging. In this case, lithium is strongly reactive and reacts with the carbon electrode to produce Li2CO3, LiO, LiOH, etc. to form a coated layer on the surface of a negative electrode. This coated layer called a solid electrolyte interface (SET). The SEI layer formed at the beginning of charging may prevent the reaction of the lithium ions with the carbon negative electrode or other materials during charging and discharging. In addition, the SEI layer performs the role of an ion tunnel and passes only the lithium ions. The ion tunnel may induce the solvation of the lithium ions, and organic solvents of an electrolyte having high molecular weight may induce co-intercalation at the carbon negative electrode, thereby preventing the breaking of the structure of the carbon negative electrode.
Therefore, to improve the cycle properties at a high temperature and the output at a low temperature of a lithium secondary battery, a rigid SEI layer is necessary to be formed at the negative electrode of the lithium secondary battery. Once the SEI layer is formed during an initial charging, the SEI layer prevents the reaction of the lithium ions with the negative electrode or other materials during repeating charging and discharging while using the battery later and plays the role of the ion tunnel for passing only the lithium ions between an electrolyte and the negative electrode.
The improvement of the output properties at a low temperature is not expected for a common electrolyte not comprising an electrolyte additive or an electrolyte comprising an electrolyte additive with inferior properties due to the formation of a non-uniform SEI layer. In addition, even when an electrolyte additive is included, in the case when the amount required thereof is not controlled, the surface of the positive electrode may be decomposed during performing a reaction at a high temperature due to the electrolyte additive, or an oxidation reaction of the electrolyte may be carried out, thereby increasing the irreversible capacity and deteriorating the output properties of a secondary battery.